The invention relates to seafloor electromagnetic surveying for oil and other hydrocarbon reserves.
Determining the response of the sub-surface strata within the earth's crust to electromagnetic fields is a valuable tool in the field of geophysical research. The geological structures associated with thermally, hydrothermally, tectonically or magnetically active regions can be studied. In addition, electromagnetic surveying, or sounding, techniques can provide valuable insights into the nature, and particularly the likely hydrocarbon content, of subterranean reservoirs in the context of subterranean oil exploration and surveying.
Seismic techniques are often used during oil exploration to identify the existence, location and extent of reservoirs in subterranean rock strata. Whilst seismic surveying is able to identify such structures, the technique is often unable to distinguish between the different possible compositions of pore fluids within them, especially for pore fluids which have similar mechanical properties. In the field of oil exploration, it is necessary to determine whether a previously identified reservoir contains oil or just aqueous pore fluids. To do this, an exploratory well is drilled to determine the contents of the reservoir. However, this is an expensive process, and one which provides no guarantee of reward.
Whilst oil-filled and water-filled reservoirs are mechanically similar, they do possess significantly different electrical properties and these provide for the possibility of electromagnetic based discrimination testing. A known technique for electromagnetic probing of subterranean rock strata is the passive magneto-telluric (MT) method. The signal measured by a surface-based electromagnetic detector in response to electromagnetic (EM) fields generated naturally, such as within the earth's upper atmosphere, can provide details about the surrounding subterranean rock strata. However, for deep-sea surveys, all but those MT signals with periods corresponding to several cycles per hour are screened from the seafloor by the highly conductive seawater. Whilst the long wavelength signals which do penetrate to the seafloor can be used for large scale undersea probing, they do not provide sufficient spatial resolution to examine the electrical properties of the typically relatively small scale subterranean reservoirs. Moreover, since MT surveying relies primarily on horizontally polarised EM fields, it is intrinsically insensitive to thin resistive layers.
Nonetheless, measurements of electrical resistivity beneath the seafloor have traditionally played a crucial role in hydrocarbon exploration and reservoir assessment and development. In industry, subterranean resistivity data have generally been obtained almost exclusively by wire-line logging of wells. There are, though, clear advantages to developing non-invasive geophysical methods capable of providing such information from the surface or seafloor. Although inevitably such methods would be unable to provide comparable vertical resolution to wireline logging, the vast saving in terms of avoiding the costs of drilling test wells into structures that do not contain economically recoverable amounts of hydrocarbon would represent a major economic advantage.
In research fields that are not of commercial interest, geophysical methods for mapping subterranean resistivity variations by various forms of EM surveying have been in use for many years [1, 2, 3, 10]. Proposals for finding hydrocarbon reservoirs using such EM surveying have also been made [4, 5] and applications to the direct detection of hydrocarbons using horizontal electric dipole (HED) sources and detectors have proved successful [6, 7].
To successfully map subterranean resistivity variations in the field of oil exploration, the orientation of the current flows induced by EM signals must be carefully considered [6]. The response of seawater and subterranean strata (which will typically comprise planar horizontal layers) to EM signals is generally very different for horizontally and vertically flowing current components. For horizontally flowing current components, the coupling between the layers comprising the subterranean strata is largely inductive. This means the presence of thin resistive layers (which are indicative of hydrocarbon reservoirs) do not significantly affect the EM fields detected at the surface since the large scale current flow pattern is not affected by the thin layer. On the other hand, for vertical current flow components, the coupling between layers is largely galvanic (i.e. due to the direct transfer of charge). In these cases even a thin resistive layer strongly affects the EM fields detected at the surface since the large scale current flow pattern is interrupted by the resistive layer. It is known therefore that significant vertical components of induced current are required to satisfactorily perform an EM survey in the field of oil exploration.
However, sole reliance on the sensitivity of vertical current flow components to the presence of a thin resistive layer can lead to ambiguities. The effects on detected EM fields arising from the presence a thin resistive layer can be indistinguishable from the effects arising from other realistic large scale subterranean strata configurations. In order to resolve these ambiguities, it is known that it is necessary to determine the response of the subterranean strata to both horizontal (i.e. inductively coupled) and vertical (i.e. vertically coupled) induced current flows [6].
Hence it is important when designing a practical EM survey for detecting buried hydrocarbon reservoirs to distinguish between source and detector configurations in which the coupling between layers is largely inductive due to horizontal currents (in which case the survey has little sensitivity to the presence of a thin reservoir) and those in which the coupling between layers is largely galvanic due to vertical currents (in which case blocking of the passage of this current flow by a reservoir leads to a survey which is strongly sensitive to the presence and boundary of hydrocarbon within the reservoir).
FIG. 1a schematically shows a surface vessel 14 undertaking EM surveying of a subterranean strata configuration according to a previously proposed method [6]. The subterranean strata configuration includes an overburden layer 8, an underburden layer 9 and a hydrocarbon layer (or reservoir) 12. The surface vessel 14 floats on the surface 2 of the seawater 4. A deep-towed submersible vehicle 19 carrying a HED antenna 21 is attached to the surface vessel 14 by an umbilical cable 16 providing an electrical and mechanical connection between the deep-towed submersible vehicle 19 and the surface vessel 14. The HED antenna broadcasts a HED EM signal into the seawater 4.
One or more remote detectors 25 are located on the seafloor 6. Each detector 25 includes an instrument packages 26, a detector antenna 24, a floatation device 28 and a ballast weight (not shown). In practice, each detector antenna 24 will generally comprise an array of antenna elements, for example, a pair of orthogonal dipole antennae elements. The detector antenna 24 measures a signal in response to EM fields induced by the HED antenna in the vicinity of the detector 25. The instrument package 26 records the signals for later analysis.
The HED antenna 21 generates both inductive and galvanic current flow modes with the relative strength of each mode depending on HED antenna-detector geometry. At detector locations which are broadside to the HED antenna axis, the inductive mode dominates the response. At detector locations which are in-line with the HED antenna axis, the galvanic mode is stronger [6, 8, 9, 10]. The response at detector locations in both the in-line and broadside configurations is governed by a combination of the inductively and galvanically coupled modes and these tend to work in opposition.
FIG. 1b shows in plan view an example survey geometry according to the previously proposed method in which sixteen detectors 25 are laid out in a square grid on a section of seafloor 6 above a subterranean reservoir 56 having a boundary indicated by a heavy line 58. The orientation of the subterranean reservoir is indicated by the cardinal compass points (marked N, E, S and W for North, East, South and West respectively) indicated in the upper right of the figure. To perform a survey, a source starts from location ‘A’ and is towed along a path indicated by the broken line 60 through location ‘B’ until it reaches location ‘C’ which marks the end of the survey path. As is evident, the tow path first covers four parallel paths aligned with the North-South direction to drive over the four “columns” of the detectors. This part of the survey path moves from location ‘A’ to ‘B’. Starting from location ‘B’, the survey path then covers four paths aligned with the East-West direction which drive over the four “rows” of detectors. Each detector is thus driven over in two orthogonal directions. The survey is completed when the source reaches the location marked ‘C’.
During the towing process, each of the detectors 25 presents several different orientation geometries with respect to the source. For example, when the source is directly above the detector position D1 and on the North-South aligned section of the tow path, the detectors at positions D5, D6 and D7 are at different ranges in an end-on position, the detectors at positions D2, D3 and D4 are at different ranges in a broadside position and the detector at positions D8 and D9 are midway between. However, when the source later passes over the detector position D1 when on the East-West aligned section of the tow path, the detectors at positions D5, D6 and D7 are now in a broadside position, and the detectors at position D2, D3 and D4 are in an end-on position. Thus, in the course of a survey, and in conjunction with the positional information of the source, data from the detectors can be used to provide details of the signal transmission through the subterranean strata for a comprehensive range of distances and orientations between source and detector, each with varying galvanic and inductive contributions to the signal propagation. In this way a simple continuous towing of the source can provide a detailed survey which covers the extent of the subterranean reservoir.
This previously proposed method has been demonstrated to provide good results in practice. However, some limitations of the method have been identified.
Firstly, since the two modes cannot be easily separated there will generally be a level of cross-talk between them at a detector and this can lead to ambiguities in the results.
Secondly, in order to obtain survey data from both in-line and broadside geometries, the HED antenna needs to be re-oriented at each HED antenna survey location. This requires the surface vessel to make multiple passes over broadcast locations and can lead to complex and long tow patterns.
Thirdly, a HED antenna based EM survey can only provide the best data possible at discrete survey locations. This is because of the geometric requirements of a HED antenna survey which dictate that, at any point during the survey, data can only be optimally collected from those detectors to which the HED antenna is arranged either in-line or broadside. At other orientations, separation of the inductively and galvanically coupled signals becomes more much difficult and data are less reliable. For instance, referring to FIG. 1b, when the HED antenna is at a point on the tow path above the detector marked D1 and on the North-South aligned section of the tow path, in-line data can only be collected from the detectors marked D5, D6 and D7, whilst broadside data can only be collected form the detectors marked D2, D3 and D4. The other detectors provide only marginally useful information at this point of the survey. Furthermore, if the HED antenna is at the location identified by reference numeral 57 in FIG. 1b, which is on a North-South aligned section of the tow path, in-line data can be collected from the detectors marked D3, D8, D9 and D10, but broadside data cannot be collected from any of the detectors. Since both broadside and in-line data are required for optimal analysis, the best data possible with the square detector array shown in FIG. 1b can only be collected from points along the tow path directly above the detector locations.